There is an increasing tendency for larger number of pixels on image sensors used in devices such as digital still cameras, mobile phone cameras and optical pointing devices, for example. Consequently, sensors have either increased in size or pixels have been manufactured smaller, or often both. The use of finer geometry technologies increase the chance of defects occurring. Typically, the defectivity of an image sensor is proportional to the area.
Defects are caused during manufacture and are usually caused by dust particles obstructing the photolithography process. Resulting defects may be open circuit or short circuit connections. If a defect occurs within a pixel during manufacture, a defective pixel is usually the result. The defective pixel can be either ignored by the user or, if the defective pixel can be identified, corrected for by interpolating between neighboring pixels. If the defect occurs on a connection that is common to either a row or column of the pixel array, then a series of pixels in the row or column may be defective, rather than a single pixel. In some cases, the entire row or column can be defective. Defects that disrupt the operation of more than one pixel are far more noticeable to a user and much harder to compensate for.
Typically, as shown in FIG. 1, a pixel array 100 has a matrix of pixels 102. Each column of pixels 102 in the pixel array 100 is connected by a common column bitline 104. Each row of pixels 102 is connected by a common row select 106. When the row select 106 is activated (set to “high”) by row drivers 108, the pixels 110 in that row are enabled for readout and the values of the pixels 102 are read out in parallel on to the column bitline 104 to readout amplifiers 110.
A horizontal scan 112 sends an “enable signal” along enable connection 114 to each readout amplifier 110 in turn. The amplified pixel value can then be read on an output bus 116. If the readout amplifier 110 is defective, the entire column of pixels cannot be readout correctly and, as such, a noticeable error occurs in the image.
U.S. Pat. No. 6,741,754 Hamilton, “Correcting for defects in a digital image taken by an image sensor caused by pre-existing defects in two pixels in adjacent columns of an image sensor”, discloses a method for correcting for defects in a digital image taken by an image sensor when there are pre-existing defects in two pixels in adjacent columns of the image sensor which causes two adjacent lines of pixels in the digital image to have corrupted data.
U.S. Pat. No. 5,436,659, “Method and apparatus for determining defective pixel location”, attempts to use digital timing techniques to identify defective pixels and store their locations for correction by an appropriate technique, such as substituting a neighboring pixel value. U.S. Pat. No. 5,291,293, “Electronic imaging device with defect correction”, utilizes redundant sensor elements for defect compensation by using a plurality of arrays and pixels in one sensor used to correct info on the other sensor.